This invention relates to a signal seeking radio for use on a motor vehicle and particularly to such a radio adapted for day and night operation. It is well known that RF signals in the AM broadcast frequency band tend to carry much farther at night due to reflections in the ionosphere. However, this can be a nuisance in the operation of a scanning or signal seeking radio on a motor vehicle. In the signal seeking operation, the radio tuner sweeps through a range of frequencies until a signal of a predetermined signal strength is detected. In the daytime, this corresponds to a clearly listenable station. However, at night, this may be a reflected skywave signal which is garbled and unintelligible due to changes in sideband symmetry, notching of the carrier or excessive co-channel and adjacent channel signals. Even good signals may be reflected from a distant station and be subject to degradation or complete disappearance as the vehicle changes position or direction or as atmospheric conditions change over time. Some prior art radios have been equipped with stop sensitivity switches, which allow the operator to choose between more or less sensitivity in the signal seeking operation. However, such switches must be manually operated, which is inconvenient and takes attention away from driving. It has further been suggested that a signal seeking radio start sweeping with the lowest sensitivity the first time across the frequency band and automatically switch to higher sensitivity with succeeding passes. However, this method of signal seeking operation is complicated and not easily understood by the vehicle operator.
It has further been suggested that the stop sensitivity of a signal seeking radio may be controlled by a timer to be decreased during those hours expected to produce night listening conditions. However, since sunrise and sunset vary over the course of the year and since some areas of the United States do and some do not go on daylight saving time for a portion of each year, such control is not adequate, by itself, to produce optimum operation in the twilight and/or sunrise times typical of, for example, commuting to and from work. It would be desirable to have a signal seeking radio with stop sensitivity automatically controlled in response to day-night condition.